(1) Field of the Invention
The present invention relates to a system for enciphering or deciphering data, and more particularly to a system for enciphering or deciphering data or messages by multiplying them by pseudorandom numbers.
(2) Description of the Prior Art
Recently, in an on-line system such as a bank system, protection of transmission data through encryption has strongly been requested in order to prevent illegal access to the transmission data and its abuse through cryptoanalysis. Not only the protection for transmission data through encryption, but also the protection for file data within a computer system has also been requested heretofore.
Of conventional data encryption systems, there are known systems in which pseudorandom numbers are added to data, as proposed for example in U.S. Pat. Nos. 3,535,642, 3,681,708, 3,691,472, 3,816,764, 3,838,259, 3,911,216. In these known systems, basically the bit patterns of transmission data and pseudorandom numbers are added together at each of the corresponding bits of the respective data and the resultant bit pattern is transmitted as a cipher message. With such system, if there is only one terminal for destination of a message, i.e., in case of an end-to-end type communication network, both sending and receiving terminals may suffice only if they have a single common stream of pseudorandom numbers as a cipher key for enciphering and deciphering. However, if the number of terminals in a communication network increases, then it is necessary to use a plurality of particular cipher keys for respective pairs of sending and receiving terminals. This results in a large number of cipher keys which must be prepared for the whole communication system.
The number of types of cipher keys may be reduced by allocating a particular cipher key for each communication channel. For instance, as shown in FIG. 1, in a system where terminals A, B, C, D and E are connected to each other via nodes P.sub.1, P.sub.2 and P.sub.3, the number of cipher keys necessary for each terminal becomes minimum on condition that different streams of pseudorandom numbers C.sub.1 to C.sub.7 are prepared for respective communication channels and enciphering and deciphering are performed by using as a cipher key C.sub.1 for the channel from A to P.sub.1, C.sub.2 for the channel from P.sub.1 to P.sub.2, C.sub.3 for the channel from P.sub.2 to P.sub.3 and C.sub.4 for the channel from P.sub.3 to B. However, if the above-noted encryption using an independent cipher key for each communication channel is employed for an encryption system where a transmission message and a stream of pseudorandom numbers are added together (EXCLUSIVE 0R operation), there arises a possibility of a cryptoanalysis of the message by performing an AND operation, relative to the same plain text, of the cipher texts obtained from a plurality of communication channels. Thus, data protection is not sufficient.
Among other encryption systems, also known is an American encryption standard system by the Department of Commerce of U.S. Government. This system however requires complicated circuits in order to perform iterative operations of combinations of substitution and transposition processings for the input data.